Crankshaft milling machines are used as conventional machining apparatuses for cutting and machining crankshafts used in engines or the like. Such crankshaft milling machines are provided with two work heads which are installed on a bed and support two end portions of a workpiece, and a cutter unit which is placed between the two work heads and machines a workpiece.
The above described cutter unit is provided with a saddle which is freely moveable in the longitudinal direction of the bed (direction of the Z axis) and a slide which is freely moveable in the direction of the Y axis (direction of the depth), which is perpendicular to the Z axis, on this saddle. In addition, a swing head of which one end portion is supported by a support axis and the other end portion swings in the up-down direction (direction of the X axis) due to a swing mechanism installed on the slide is mounted on the slide.
A cutter drum which is rotated by a cutter motor is provided in the above described swing head, and a cutter is mounted on this cutter drum via a cutter adaptor. This cutter is formed of a main cutter body in ring form and a number of cutter chips which are attached in the inner periphery portion of this main cutter body with intervals in the direction of circulation, and some of these cutter chips create the outer peripheral surface of the journal of a workpiece, while others simultaneously create trenches in the two end portions of the journal of a workpiece and a journal thrust surface which continues to the respective trenches and crosses the outer peripheral surface of the journal at a right angle.
In addition, in such conventional crankshaft milling machines, the cutter itself is rotated on the basis of machining data inputted in a control apparatus (NC apparatus) attached to the main body of the machine, and at the same time, the cutter is rotated around the journal of the workpiece through movement of the cutter in the direction of the X axis and in the direction of the Y axis so that a cutting process is carried out on the workpiece, and thus, the outer peripheral surface of the journal of the workpiece and a journal thrust surface which crosses the outer peripheral surface of the journal of workpiece at a right angle are created. Here, when the cutting process is carried out, the workpiece is sandwiched between work rests so as to be prevented from vibrating during the cutting process.
Incidentally, the precision required for the circularity on the outer peripheral surface of the journal is high in the machining of a crankshaft. In addition, in recent years, high precision has become a requirement for the straightness on the journal thrust surface which makes a right angle with the outer peripheral surface of the journal. In the above described conventional crankshaft milling machines, however, the workpiece slightly bends, even though the workpiece is prevented from vibrating by the work rests during the cutting process, and therefore, it is difficult to increase the circularity on the outer peripheral surface of the journal and the straightness on the thrust surface of the journal.
The precision in the circularity on the above described outer peripheral surface of the journal can be increased using, for example, a control apparatus for a crankshaft milling machine as that proposed in Patent Document 1 (Japanese Patent No. 2691894). In this control apparatus, the amount of feed of the cutter relative to the outer peripheral surface of the journal is controlled on the basis of corrected machining data after the machining data is corrected on the basis of data for correcting the circularity, which is gained by comparing the results of measurement of the machining dimensions of the outer peripheral surface of the journal and the target machining dimensions in a workpiece where a cutting process was carried out with the cutter in advance, and therefore, error in machining caused by bending of the workpiece can be reduced, and thus, such effects are gained that the precision in the circularity on the outer peripheral surface of the journal can be increased.
However, the technology for increasing the precision in the circularity according to the above described Patent Document 1 cannot be applied as a means for increasing the precision in the straightness on the thrust surface of the journal as it is. Here, as a means for increasing the precision in the straightness on the thrust surface of the journal, a means for taking apart and adjusting the machine, for example the cutter unit or the work heads provided on the bed, for correction can be cited, but in such means for correction, 1) a great amount of labor and time are required, 2) change in the type of the workpiece and the like cannot be dealt with, 3) correction cannot be made for every machined portion and so forth, and therefore, it is practically impossible to use a means for correction by taking apart and adjusting the machine in this manner, and thus, there is a problem, such that the precision in the above described straightness cannot be increased.
The present invention is provided in view of these problems, and an object of the invention is to provide a method for machining a crankshaft according to which the precision in the straightness on the thrust surface of the journal relative to the outer peripheral surface of the journal can be increased, an apparatus for machining a crankshaft, a control apparatus and a program.